The aircraft were on a mission as part of the NATO-led International Security Assistance Force. The OH-58 Kiowas never intruded on Pakistani airspace, Pentagon spokesman Bryan Whitman said.

The Pakistani fire did not hit the choppers and the American aircraft did not return fire, Whitman said. “U.S. forces always maintain the right of self-defense,” he said. “In this particular case, my understanding is they just broke off and didn’t return fire.”

The incident occurred near the Pakistan border in the Afghan province of Khowst, NATO officials in Kabul said. NATO officials are speaking with Pakistani military officials to get to the bottom of the situation.

“Over the past several weeks and months, we’ve been doing things to enhance this coordination along the border,” Whitman said.

NATO, Afghanistan and Pakistan are working together to establish coordination centers along the border. One already is operational in Nangarhar province at Camp Torkham. The centers will work to prevent misunderstanding among the three entities, officials said.

“I think it’s encouraging that at the time this incident occurred, we were immediately able to contact Pakistani authorities and determine the origin of the fire,” Whitman said. “We’ll have to work through how we prevent incidents like this in the future.”

Al-Qaida and Taliban insurgents seek to exploit the border area where the terrain is difficult and it is easy to make mistakes, NATO officials said. However, the Taliban do not have helicopters, so it is unclear what message Pakistani troops believed they were sending, officials said. This is the first time Pakistani soldiers have fired on coalition aircraft, they said.

The Pakistani embassy had no immediate comment, Whitman said. Pakistan remains an ally and a good partner in the global war on terrorism, he said.